Sputtering is a preferred industrial thin film coating process. In this process, a target material is deposited over a substrate area. By bombarding the target with gas ions accelerated by a high voltage, target atoms are caused to eject, or sputter, from the surface. Target particles then traverse the sputtering chamber and are deposited onto the substrate as a thin film.
For some sputtering applications, it is in several ways favorable if a large fraction of the sputtered atoms is ionized. Firstly, the sputtered ions might be attracted to the substrate by applying a bias to the latter. This will add energy to the growing film, which is beneficial for the film growth. Secondly, the attracted ions will have a preferential perpendicular direction when arriving onto the substrate surface which enables deposition in groves and trenches. Thirdly, if the fraction of ions is sufficiently large, it may be possible to run the process in a self-sustained mode. This means that ions from the target material sputter themselves without addition of an extra (inert) gas. This will of course result in a much cleaner process where no inert gas species is contaminating the deposited film.
The fraction of ionized sputtered species is correlated to the target ion current density. A higher current density implies a larger fraction of ionized species. Generally, the maximum tolerable current density is limited by the efficiency of the target cooling system.
By adding a reactive gas to the sputtering process, it is possible to reactively sputter thin films consisting of oxides, nitrides, carbides etc. The reactive sputtering process has found widespread applications, in for example coating of tools, decorative coatings, window glasses, plastic webs, electronic components, data-storage components etc. Due to its highly complicated behaviour, the reactive sputtering process is associated with a number of difficulties. The process usually exhibits a behaviour presenting a hysteresis effect, which makes it difficult to control. Moreover, the deposition rate of compound is usually much lower than the deposition rate of pure metal (sometimes as much as 10-20 times lower). Finally, deposition of insulating compound thin films from metal targets implies charging and subsequent arcing at the target.
The only way to avoid hysteresis in the reactive sputtering process, so far, has been to increase the external pumping speed of reactive gas. This may sometimes be realized for small systems but leads to unrealistically high pumping speeds in large industrial systems.
In an ideal controllable process, reactive sputtering would be carried out from a clean metal target and the metal atoms would react with the gas when arriving at the substrate. It would then be possible to change the compound concentration in the deposited film by the reactive gas flow. Unfortunately, it is not possible to obtain these ideal conditions because the reactive gas also reacts with the target, resulting in compound formation at the target. Experiments have previously been carried out in order to introduce a pressure gradient in the processing chamber and thereby reduce compound formation at the target. Such a gradient is not easily obtained and the impact on the process behaviour has so far been quite small.
To solve the problem with charging and arcing during deposition of insulating films, it is possible to make use of specially developed power supplies which add extra positive pulses in order to neutralize the target.
Another way to partly overcome this problem is to increase the total gas pressure so that a substantial fraction of the sputtered metal from the high-erosion parts of the target scatters back onto the low-erosion parts of the target, thus making this part more metallic which suppresses charging and subsequent arcing.
The concept of using a magnet that is moving relative to the target to induce a moving erosion area is well known and described in several patents and patent applications, see for example U.S. Pat. No. 6,183,614, WO 01/23634 and WO 92/02659. In U.S. Pat. No. 6,183,614, asymmetric magnetic fields are introduced in order to achieve advantageous high-density plasma sputtering. WO 01/23634 uses a plurality of magnets to induce a magnetic field having a predefined arbitrary shape and thereby improve material utilization. An alternative target set-up is disclosed in WO 92/02659, which employs a cylindrical magnetron for the purpose of reducing arcing.
Problems associated with prior art sputtering processes remain and there is a strong demand for an improved sputtering method.